<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        /**
         * @param {number} num
         * @return {number}
         */
        var addDigits = function (num) {
            let arr = String(num).split('')
            let newArr = arr.map(item => parseInt(item))
            let sum = newArr.reduce((a, b) => a + b)
            if (sum < 10) {
                return sum
            } else {
                addDigits(sum)
            }
        };
        console.log(addDigits(38))
    </script>
</body>

</html>